An elevator installation usually comprises an elevator car and at least one counterweight, which are moved in opposite sense in an elevator shaft. The elevator car and the at least one counterweight in that case run in or along guide rails and are supported by at least one support means, which is guided over a driving drive pulley. The support means usually consists of one or more sheathed steel cables, one or more synthetic fiber cables, one or more flat or profiled belts (wedge-ribbed belts) or a parallelly extending composite of the respectively mentioned constructions, in which each individual support means can be guided over a respective individual drive pulley.
It is possible with such modern support means to realize such a high level of traction on the driving drive pulley that, for example, the elevator car is raised further although the counterweight could be obstructed in its downward movement by an unforeseen jamming in the elevator shaft or by an unforeseen seating on the shaft floor buffers. The same problem can arise with the counterweight if the elevator car should sit on the shaft floor buffers. This lifting up of a load—be it the elevator car or the counterweight—at one side of the drive pulley without the provided counter-load at the other side of the drive pulley running conjunctively to freely rise is impermissible and can lead to dangerous states (dropping back of the elevator car or the counterweight).
Accordingly, monitoring devices for detection of a relaxed, slack support means have been developed. They are based, as disclosed in, for example, European published specification EP-A1-1 953 108, on a spring-reinforced mounting of the overall drive and a deflecting unit with at least two further rollers for the support means.
A device for stopping falling is known from the document WO 2006/082460, in which a support means breakage prevents crashing down of the counterweight. The device for stopping falling is mounted at the counterweight and triggers arrest of the counterweight as soon as a force threshold is fallen below. If in the solution according to WO 2006/082460 fracture or loosening of the support means occurs then a resiliently mounted axle is displaced, which triggers the device for stopping falling.
A sensor device is known from the document DE 10 2006 027989 A1, which monitors individual support means, here specifically chains. The corresponding sensor device is independent of the drive elements, deflecting rollers and other load-bearing parts of the described elevator installation.
The document FR 2 618 420 discloses a device for monitoring an individual support means at its deflecting roller, wherein in the case of slackness of the support means a switch is actuated which switches off the drive. The device according to FR 2 618 420 thus monitors the slackness of an individual cable and not the slackness of the entire support means run. In the case of occurrence of a slack support means there is intervention in the control of the elevator car by way of an electrical contact.
International published specification WO-A1-2007/144456 thereagainst discloses a direct fastening, which is also spring-loaded, of the support means. A relaxation of the spring thus occurring at the fixed fastening point of the support means due to load relief thereof triggers a switch which switches off the drive. It is disadvantageous with this solution that it is suitable only for elevator installations with a small conveying height and that there is no detection at the directly affected support means sections between drive pulley and support roller of the elevator car or between drive pulley and support roller of the counterweight.
The disadvantages of these two solutions according to the prior art are on the one hand the constructional outlay and on the other hand the difficulty of detecting a reliable trigger value. Particularly in the case of large conveying heights and thus lengthy support means sections the high intrinsic weight of the support means emerges as unfavorable inasmuch as a difference between loaded operating state of the support means and slack state of the support means is detectable only with difficulty.